Structural archaeologist Geoff Carter's radical view of building in the ancient world, especially the archaeology of the lost timber built environment of Southern England. It is new research into of prehistory of architecture, available in a series of articles that are designed to be read in order, and to be accessible to the non-specialist - and there is even some humour

31 January, 2016

In response to the latest blog Carnival organised by Doug
Rocks-Macqueen, the champion of archaeological blogging, over at Doug’sArchaeology, I am posting about the challenges of modelling a prehistoric roof
structure in 3D.

The story so far…

My work is based on the idea that archaeological buildings
are mathematical structures which can be detected and understood using the same
principles that underpin the engineering of the built environment.

As regular readers will know, I have had the misfortune to
have discovered how, in theory, the large Neolithic / EBA structures
represented by postholes known as Class Ei buildings [1] worked, at least in
plan and section. The next stage is to
model the structure in 3D to understand its assembly; the initial challenge is
finding an appropriate starting point, since the value of everything else, and
many man hours is dependent on this decision.
What is also challenging, at least in an abstract sense, is that the Ei
building I am currently modelling at moment is Stonehenge, the well-known
ritual monument and mystery at heart of British faith-based archaeology.

While I have established, by what might be called
conventional means, that these arrangements of postholes represented the
footprint of buildings, listing 12 fairly straightforward observations that
demonstrate this, it was necessary to develop a more complex understanding to
explain precisely how these posts might support a roof.

I called this structural system Interlace Theory; just how
complex it is, may be judged by the fact that I first grasped the principle
almost exactly 6 years ago, [although to be fair on myself I have been
otherwise engaged during most of the intervening period]. Woodhenge has been my principle case study
because it is by far the best data set, however, Stonehenge is a much simpler
structure and it has an elevation which can serve as the necessary starting
point.

A tale of two analogiesWhile I am happy to discuss a myriad of minor matters arising from interlace theory, it would be a somewhat monotonous monologue, - you know the bit after you tell the taxi driver that you are not aware of a secret room under the sphinx - so I will find a different way of explaining this stage of the research.As an archaeologist, I might find pieces of kitted fabric, and while it is possible to reverse engineer the process of knitting, how you construct garments out of a fabric is a very different question. Just like buildings, garments vary in pattern, size and style, which cannot be fully extrapolated from the basic technique. Interlace theory explains how the structural might fabric work, but not necessarily how it was used to construct a particular building; as with garments, this involves different and more flexible rules.However, if we find a piece of a tailored cloth then it might be possible to reconstruct the type of garment, or, a better analogy might be finding a piece of metal, which luckily it is recognised as the base plate from a watch. Each of the holes in the plate correspond to the location of a component, which, with a knowledge of watch-making, you could reverse engineer the mechanism. However, while you could demonstrate it was part of a watch by modelling how it worked, what it actually looked like would not necessarily be apparent. Regrettably, If you do not understand horology it will remain a ritual object.

Doin’ the math

While watches are complicated, they are small compared to a building, which may have simpler engineering, but a lot more components, creating its own complexities for modeller and builder alike. While a modern building might get away with a hundred standard mass produced components, most of the components in a prehistoric structure are bespoke and made to fit, which is time consuming and resource intensive. It is also worth noting that most drawing systems are set up of assumption you will be modelling rectilinear structures, in which for example, the rafter pairs are at right angles or parallel to the main axis. By contrast, in a circular building the rafters are radial and all at different angles relative to whatever serves as the principle axis. We can do a simple guestimation to demonstrate how complex roofing a structure the size of Stonehenge might become; just like the builder you have to work up from the lowest components, which in terms of the roof is the outer edge represented by the 30 Y posts. The footprint of a building is was not circular, neat or regular, it is 30 sided, an irregular Triacontagon; to cover it would take the equivalent of 4 roofs with a combined width of 180’, which, if we want a rafter every 6’ along the edge need about 90 rafter pairs or 3 supported per Y post. Each pair has a tie so that’s 3 per post along with the 3 horizontal plates or elements, which is actually 6 because they run between 2 Y posts. So that is 9 horizontal components per post, which we could double if a clerestory or widows are indicated, which is why with a guestimated 270 or 540 components to be attached to the 30 Y posts and fitted together, before we reach the 90 or 180 pairs of rafters which form this outer part of roof, which is why where you start is so important. However you figure it, it’s is going to be a complex challenge to assemble such a structure successfully so that components don’t pass through each other, which is why Interlace Theory was necessary. It is also why I am working on Stonehenge, since structures are supported at their lowest point, the stone components particularly the stanchions [trilithons] give us a significant clue to the position of the lowest parts of the structure and a starting point. In the model I have created and positioned 90 individual ties, now the challenge is too fit them together systematically, this could take some time.

Snapshot from a 3D model of a ritual monument showing 90 roof ties.

The Challenges of reverse engineeringStrange as it might seem, it is important not to think about what the structure looked like, but while it is actually hard not to speculate, and if pressed I could tell you all sorts of things about this type of roof, the problem is that I would almost certainly turn out to be wrong, and this is not about guess work. Reverse engineering an archaeological structure is an evidence based deductive process, so, in an ideal world, it is not;

how I would have done it;

the best way of doing it;

the simplest way of doing it;

how someone else does it, or did it, whether locally, or on the other side of the world.

Problematically, the latter is a basic building block of much scholarship which considers plan “shape” to be of primary importance regardless of form, function, scale, technology and cultural or environmental context.This in turn has resulted in a visual culture of the past for which there is no real evidence. This has given rise to the biggest challenge faced by structural archaeology - peoples' imagined “pictures” of the past; being told that Stonehenge was a largely wooden building is a bit like trying to imagine a different coloured Jesus. Post University Challenge.It is often said that employers or institutions want people who can think for themselves, while what they actually want is people think like themselves. Universities invest in and monetise accepted ideas, thus new ones are not welcome, as they disrupt the market, and so research that does not support the existing narrative is of no inherent value.In later periods people dug postholes for buildings, and pits to shit in, but apparently, in the British Neolithic they did both to communicate complex ideas to supernatural forces and post-processualists. Thus, while research into “...how postholes served as focus for structured deposition in a wider ritual landscape..” could be funded and even peer reviewed, evidence based reverse engineering will remain way too challenging for Universities, but freely available in the blogosphere.. . . and on a lighter noteAs a gesture to the Gods indicating we intend to try and keep on going, TSA has rescued a new member of staff; Tiny is in charge of destructive testing, spatial redistribution, disciplining the printer, and pest control. This year, I was trying to teach him structural archaeology, but his responses are a bit incoherent, and he is still coming to terms with the concept of gravity; however, he is young, so we have decided it would be best if he concentrated on his master’s degree in post-processual thinking, at least until his brain develops further.Source[1] Neolithic houses in northwest Europe and beyond’ (1996) (Oxbow monograph 57) [Paperback]. T.C. Darvill (Editor), Julian Thomas(Editor) [[Fig 6.9]].

13
comments:

James Sartain
said...

An architect named Sarah Eubank has also posted a theory that Stonehenge was a building over on the Ancient Origins site. They had it flagged as a radical new theory. I left them a comment that you had the theory quite some time ago. Thought credit should go where credit is due.

Hi JamesThanks for upholding the honour of TSA, I hope pointed out that this theory extends to all timber structures in this class, not just the wyrd one with a stone supporting wall and stanchions.The architect Inigo Jones had concluded it was a building [ a Roman temple], in a book published posthumously in 1655.

Here's somebody else with a similar idea for a wooden roof over Stonehenge. His goes to a point and looks like a wigwam.http://www.stonehenge.tv/roof.html

Couldn't it have been open at the top, like the Pantheon?

Apart from the little purple sketch indicating a ridged circle around a peak, it is hard to visualise your idea. Is that complication of poles the actual roof shape, or just a basis supporting a ridge somehow? Are the poles actually plaited, (or jointed) consistently over and under,like the Celtic interwoven designs or a basket,forming some sort of self supporting mat at an angle to the ground? Is the centre open or is there a roof over that, supported somehow on the lower level roof? How about a nice dome? No glass for windows? Thatch or shingle or slate roof? Birds flying through or nesting inside? The ancestors of your cat might have enjoyed chasing birds and mice there, if they'd got there early enough!

On another topic, did I correctly see a comment on one of your older threads, suggesting that Seahenge is a hoax?

I've been working on interwoven domes for decades, never thought of Stonehenge as one. Pygmy ladies (the most elegant architects on earth, bar none) ground their domes with 10 shallow postholes supporting a woven hemi-icosahedron.

Hi OR,Thanks for the comment; this is reverse engineering, it is not about visualisation, I can't picture iteither yet - but I don't want to.Seen that wigwam idea; projected not deduced. The roof will cover the whole thing, buildings with holes in are not common in UK. Thatch or shingle, windows are openings for light and air, it would be thousands of years be four glass would be regularly used in windows, until then they had to make to with shutters.About Sea Henge - it is meant to look like a tree thrust into the ground, which it is not, it's only a short section of trunk; So what I was suggesting it was a faked "religious" to accord with a legend or the presence of giants - that sort of thing. [It could be an excarnation site - not sure of the evidence to support that].

Well, I'm not claiming Stonehenge was domed, but it certainly could have been, without technical difficulties nor giants. A portable climbing scaffold, lots of coppiced & trimmed flexible branches and binding cord, and a crew would probably get it done comparatively swiftly.

A Pygmy lady pairs each wicker post with the opposing one, at the apex, bending and binding them (about 4.5'/1.5m) to make one long arched pole, so a sum of 5 interwoven structurally, with a pentagon sky-window, then interweaves additional twigs to make a latticed dome overlay, then inserts leaves around in a coil to waterproof it. Takes 2 hours, one if hurried, and helped by a friend.

The length of a "pole" from posthole to posthole would be: ?Multiply by scale factor: ?

Without the leaf shingles, the lattice sky grid, with an additional fine net-grid of flax thread, would provide an excellent observatory, cosmic movements and supernova identified by an arm-chair scholar to the nearest inter-net position.

Want me to build one? Probably need a permit? I'ven't any gold, so perhaps some donation requests to financial giants would be necessary... ?

Crowd funding?The minimum span is about 22.5', which for a hemispherical done, allowing for a 3' straight section at the bottom, would require [[[22.5 x 2] x [22/7]]/ 4] + ] = about 36'; so the smallest sapling required would be sapling to 36'; good luck bending a 40' tree. It would give 3200 square feet of roof per little dome; you would need 8 tons [American] of thatch per dome, but if can bend 40 ' trees it would probably be OK - till it snowed.

I' ll start making enquires about planning permission right away!

[a single dome made from bending bending 140/ 150' saplings is about 51,000 square feet in area, which might weigh a quite bit].

Excellent idea! Crowd funding. Wonder how to get the ball rolling, something like kickstarter.

Thatch? Wouldn't that block the view of the stars and supernovas? I was thinking of a (sort-of) "transparent" dome with 360 degree vision and grid of squares surrounding the observer (Druid? Wizard? They always wore long robes, it got chilly at night and smokey fires would ruin the view, perhaps had an apartment below the dome's upraised floor (which I guess sat on the stone circle), to map the heavens, a Neolithic astronomic observatory. Regarding planning permission, yes, good, might be a bit messy without that.

It may be indicative of (something or other) that the height difference between the top surfaces "front" trilithons and the taller ones surrounding the Altar Stone is pretty much the thickness of the butt end of a decent length of oak tree (if we're piling up our Spillikins like that, just to see what happens.

In a later post I will address what this is all about in more technical terms, this is really just a teaser to show I am still at work.

However, more generally, the issue of taper is an interesting one, as it impacts model building in several significant ways.I used to make components by starting with a tapering trunk [based on yield tables], which is fine for posts, but becomes problematic for horizontal components where one surface has to “level”.There is also an important observation about this class of structure; postholes decrease in diameter towards the centre, which implies that ties were arranged with taper towards the middle. Any system of timber architecture has to deal with this issue, but for modelling one has to assume that horizontal roof components are level, thus I use uniform components for this type of work. This does not have to apply to ties that could be left in the round. For practical purposes one has to theorise the use of boxed heart from very large trunks as the main source of horizontal timber.

I really don't know, perhaps a seasonal structure, annually rebuilt upon the stone & timber base? I'm thinking also of the Gobekli Tepe structure being of a similar type, as the carvings on the pillars there may have linked constellations to local fauna, and perhaps the Newgrange symbols as well. http://www.pineroloplay.it/wp-content/uploads/2014/02/gobekli-tepe.jpg http://www.kerryexperiencetours.ie/wp-content/uploads/2014/10/Newgrange-Ireland.jpg

Ok, that was fun, but now back to your Interlacing Theory. (hehe)

Note: Regarding your observation that some "square" based huts weren't very square: in the Malay Archipelago by Alfred Wallace, he mentions that on some islands people constructed square huts without diagonals, instead inserting very crooked poles which gave similar support, though eventually the huts would be windbeaten and begin to tilt precariously. I don't think the postholes would indicate this arrangement.